Ink jet printers having one or more ink jet heads for projecting drops of ink onto paper or other printing medium to generate graphic images and text have become increasingly popular. To form color images, ink jet printers with multiple ink jet printing heads are used, with each head being supplied with ink of a different color. These colored inks are then applied, either alone or in combination, to the printing medium to make a finished color print. Typically, all of the colors needed to make the print are produced from combinations of cyan, magenta and yellow inks. In addition, black ink may be utilized for printing textual material or for producing true four-color prints.
In a common arrangement, the print medium is attached to a rotating drum, with the jet heads being mounted on a traveling carriage that traverses the drum axially. As the heads scan paths over the printing medium, ink drops are projected from a minute external orifice in each head to the medium so as to form an image on the medium. A suitable control system synchronizes the generation of ink drops with the rotating drum.
To produce images of certain colors, more than one color of ink is combined on the medium. That is, ink drops of a first color are applied to the medium and then overlayed with ink drops of a second color to produce the desired color of the image. If the drops do not converge on the same position on the medium, that is, the drops of the two colors do not overlay one another, then the color of the image is distorted. Furthermore, it is also important that drops of substantially uniform size and shape be generated by the ink jet heads. To the extent that the drops are non-uniform, the image is distorted. This distortion affects the clarity of textual images as well as of pictoral images.
In one basic type of ink jet head, ink drops are produced on demand. An exemplary drop-on-demand ink jet head is illustrated in U.S. Pat. No. 4,106,032 of Miura, et al. The Miura et al jet head has a two compartment ink chamber comprised of an inner horn compartment and an outer ink compartment which communicate with one another through a connecting channel of restricted cross section. Ink is delivered to the outer ink compartment of the ink jet head. Whenever a drop of ink is needed, an electric pulse is applied to a piezoelectric crystal, causing the crystal to constrict. As a result, because the crystal is in intimate mechanical contact with ink in the horn compartment, a pressure wave is transmitted through the ink chamber. In response to this pressure wave, ink flows from the outer ink compartment and through an ink orifice passageway in an ink chamber wall and forms an ink drop at an internal ink drop-forming orifice outlet located at the outer surface of the ink chamber wall. The ink drop passes from the drop-forming orifice outlet and through an air chamber toward a main external orifice of the ink jet head. This latter orifice is aligned with both the internal orifice and the connecting channel and also leads to the printing medium. Air under pressure is delivered to the air chamber and entrains the drop of ink in a generally coaxial air stream as the ink drop travels through the air chamber. This air stream increases the speed of the drops toward, and the accuracy of applying the drops to, the print medium.
The only prior art ink jet heads capable of stable operation at an ink drop generation rate of up to twenty kilohertz have such a two-compartment ink chamber in combination with an air assist. However, there are a number of drawbacks associated with a two-compartment ink chamber design. For example, they are relatively expensive to fabricate. In addition, it is difficult to align the connecting passageway, internal ink drop-forming orifice outlet, and main external orifice outlet of this type ink jet head. Furthermore, the connecting passageway can become clogged with contaminants and, because of its internal location and size, is difficult to clean. In addition, it is very difficult to remove air bubbles from the inner horn compartment through the connecting passageway to purge such bubbles from the ink jet heads. Bubbles can interfere with the drop ejection performance of the jet head. Also, in implementations of the Miura et al type ink jet head known to the present inventors, relatively high drive voltages (i.e. 180-200 volts peak to peak) must be applied to the actuator in order to generate ink droplets, particularly at higher drop repetition rates. Consequently, drive transformers and other circuit complexities are introduced in order to obtain drive signals of this magnitude.
Another form of air assisted ink jet nozzle is disclosed by W. L. Dollenmayer in an International Business Machines Technical Disclosure Bulletin, Vol. 22 No. 6, pp. 2333-34 published Nov. 9, 1979. This bulletin discloses a single compartment ink chamber within a nozzle. The ink chamber has an ink outlet which is surrounded by a secondary air nozzle outlet which receives low pressure air. The air nozzle and ink outlets terminate in the same plane. Therefore, ink from the ink nozzle outlet does not pass through an air chamber and then through another orifice as in the case of the Miura et al design. The ink jet nozzle of this reference operates at a relative low typical maximum drop repetition rate, (i.e. 4 to 6 kilohertz). Relative higher air velocity, which would allow a higher drop repetition rate, cannot be used in this design. In such a case, the air would tend to spontaneously pump ink from the ink nozzle regardless of whether a pulse is applied to the ink. This would result in the generation of ink droplets at undesired times.
Another prior art ink jet head, developed by NEC is disclosed in the Dec. 5, 1983 issue of "NIKKEI Electronics." This ink jet head utilizes a cylindrical piezoelectric element which expands and contracts in response to driving signals. When the element contracts, an ink chamber surrounded by the element is squeezed to eject a drop of ink from a conical nozzle. Ink passes through a rectifying valve to the ink chamber and a fluid resistance element is placed at the nozzle side of the ink chamber. A larger fluid resistance is present at the nozzle side of the resistance element than at the rectifying valve so as to prevent reverse flow of ink through the chamber. Also, this article has one figure which appears to disclose a nozzle with a tip inserted partially into an opening through a plate. Air flows along the surface of the nozzle and through the opening through the plate.
This NEC ink jet head has a number of drawbacks. The use of valve and resistance elements adds manufacturing complexities. Also, the NIKKEI Electronics article mentions problems in driving the disclosed ink jet head above five kilohertz without the rectifying valve. Also, drop frequencies seem to be limited to about ten kilohertz, even with the valve. In addition, relatively low air and ink pressures are apparently employed as the air flow is understood to move at approximately the speed of the ejected ink drops. Consequently, the air does not significantly accelerate the generated ink drops. Furthermore, in a design in which the nozzle tip is inserted into an opening through a plate, the air flow, if increased in velocity, would tend to pull ink from the nozzle tip even in the absence of a pulse from the piezoelectric element. This would result in the ejection of undesired ink drops.
U.S. Pat. No. 4,380,018 of Andoh et al., at FIG. 18, discloses still another form of air assisted ink jet head which has a two compartment ink chamber. Thus, this device suffers from drawbacks similar to those discussed above in connection with the Miura et al. patent. In addition, the Andoh et al. patent, as well as U.S. Pat. Nos. 4,549,188 of Shackelton, 4,312,010 of Doring, 4,518,974 of Isayama and 3,940,773 of Mizoguchi et al. disclose a variety of non-air assisted ink jet heads with single or the double compartment ink chambers. For example, FIG. 9 of the Shackleton patent shows a single ink chamber non-air assisted ink jet head having a first section of a first diameter and a second section, adjacent an orifice outlet, of a reduced diameter. Non-air assisted ink jet heads suffer from a number of drawbacks when compared to air-assisted heads, primarily in the fact that such non-air assisted heads apply drops of ink to printing medium at limited frequency rates, such as from four kilohertz to six kilohertz.
Therefore, a need exists for an improved air-assisted drop-on-demand ink jet head which is directed toward overcoming these and other disadvantages of prior art devices.